A high pressure turbine of a gas turbine engine is surrounded by an annular turbine shroud, and a small annular gap is defined between the outer tips of the turbine rotor blades and the opposing inner circumferential surface of the turbine shroud. Typically, a turbine shroud is formed by a plurality of arcuate shroud segments combined into an annular assembly, and attached to an inner peripheral wall of a turbine casing. See Japanese patent laid open publication No. 4-330302 and Japanese patent laid open publication No. 2000-54804, for instance.
A turbine shroud is exposed to combustion gas of a high temperature, and this causes a temperature gradient in a radial direction. The temperature gradient in turn causes an uneven thermal expansion of each shroud segment in such a manner that the shroud segment warps in a direction to reduce the curvature radius thereof.
Each shroud segment is typically provided with a hook portion, and the opposing inner circumferential surface of a turbine casing is provided with an annular axial slot opening out in an axial direction. The hook portion is provided with an axial wall that is received in the axial slot, and this secures the shroud segment in position relative to the turbine casing.
The hook portion, along with the main body of the shroud segment, undergoes a thermal expansion as the engine is warmed up. To avoid the thermal expansion of the hook portion from causing undue thermal stress, a prescribed clearance is defined between the outer circumferential surface of the axial wall of the hook portion and opposing inner circumferential surface of the annular axial slot. However, this clearance causes leakage of cooling air from a cooling air chamber defined around the turbine shroud and the opposing surface of the turbine casing into the turbine chamber, and this may impair the performance of the gas turbine engine. In a gas turbine engine, a slight drop in engine performance means a serious problem for fuel economy.